1. Field of the Invention
The invention relates to digital magnetic tape transport unit and more particularly to a configuration which allows a high performance self threading tape transport unit to be packaged within a standardized relatively small mounting rack.
2. Prior Art
The use of high performance magnetic tape transport unit for feeding magnetic tape through a processing station so as to read digital data from the magnetic tape or to write digital data onto said magnetic tape is well known in the prior art. Prior art magnetic tape transport units are packaged in a parallelogram type mounting rack. The tape transport unit comprises a machine reel and file reel mounted to the mounting rack so that the axis of rotation for each reel is on a straight line parallel to and adjacent to one reference side or reference edge of the mounting rack. One or more drive capstan and a processing station, which usually includes a read/write head, are positioned on the side of the straight line farthest away from the reference side or reference edge. A pair of vacuum chambers are positioned one on each side of the processing station with major axis running in a direction parallel to the length of each vacuum column and perpendicular to the straight line.
In order to satisfy the high data throughput, (be it reading and/or writing) for the high performance tape transport unit, the magnetic tape is transported intermittently and bidirectionally at relatively high speeds past the read/write head, by the capstan. In order to minimize the adverse effect of the reels inertia on the rapidity with which the capstan moves the tape an optimum length or loop of tape is maintained in each of the vacuum chambers. By isolating the inertia of the reels from the drive capstan the effective mass which the drive capstan is constrained to move, is the mass resulting from the effective length of tape extending from the midpoints of the vacuum chambers and through the processing station.
Due to the rapidity with which the capstan moves the effective length of tape, the vacuum chambers are constrained to be designed with an optimum length. The length is such that the capstan will not pull the tape out of the vacuum chamber during system operation. To meet the optimum length requirement the mounting frame or rack of prior art high performance tape transport units are usually rectangular with the vacuum chambers running parallel to the long sides of the rectangular frame. A more detailed description of configurations for prior art high performance tape transport units are given in U.S. Pat. Nos. 3,057,568 and 3,057,569.
Although the prior art high performance tape transport units function satisfactorily for their intended purpose, these devices tend to be relatively large and, therefore, occupy an unusual amount of space. The relatively large size of prior art tape transport unit is the result of the linearly spaced machine and file reels coupled with the relatively long vacuum chambers which are positioned so that the major axis for each chamber is perpendicular to the line joining the axis of rotation for each reel.
Several attempts have been made in the prior art to arrange the necessary elements of a high performance tape transport unit to fit a standard industry size mounting frame.
In one of the prior art arrangements a machine reel and a file reel are positioned so that the axis about which the reels rotate are located on a straight line which is parallel to one edge of the support frame. A relatively straight line tape threading path with a plurality of tape guides are positioned below the reels. A magnetic head is positioned on one side of the tape threading path while a capstan is situated on the opposite side of the tape threading path. A pair of vacuum chambers are asymmetrically disposed relative to and on the capstan side of the tape threading path. The vacuum chambers have similar but differing inclination to provide maximum chamber length as well as sufficient wrap on the capstan to prevent slippage. Although this device is an improvement over the prior art device in that the size of the tape transport unit is reduced it has one drawback. Particularly, this configuration does not lend itself easily to automatic threading due to the fact that the magnetic transducer is positioned within the tape threading path. A more detailed description of the aforementioned device is given in U.S. Pat. No. 3,645,472.
In another type of prior art configuration aimed at minimizing the size of the mounting frame of a high performance tape transport device the essential elements (for example, capstans, guides, vacuum chambers, and magnetic heads) are arranged in two parallel planes. The transport consists of a mounting panel upon which the essential elements are mounted. The principal operative path of the tape transport is positioned in a first predetermined plane adjacent to and at right angles to the surface of the mounting panel. The essential elements in the principal operative path consist of a magnetic head and a first pair of guide rollers, positioned one on each side of the magnetic head and arranged parallel to one side of the mounting panel. A pair of vacuum chambers are arranged in a V configuration with the closed ends together and the open ends lying one on each side of the first pair of guide rollers along the principal operative path. A machine reel and a file reel are mounted with the rotational axis for each reel position in horizontal alignment and parallel to one side of the mounting panel. The machine reel and the file reel lie in a second predetermined plane which is super imposed above and parallel to the first predetermined plane. The orientation of the reels is such that the rotational axis for each reel is outside of the V defined by the chambers but the vacuum chambers are overlapped by the reels. Translational guide rollers are used to guide the tape from the first plane to the second plane. A more detailed description of this system is given in U.S. Pat. No. 3,380,682.
Although the aforementioned types of devices reduce the size of the prior art tape transport unit, it does not lend itself to automatic (i.e., self) threading. The reason is that the tape threading path is circuitous and complicated. Moreover, the overall cost of the system tends to be relatively high due to the use of translational guides and dual capstans.
In yet another type of prior art configuration, compactness is achieved by mounting a machine reel and a file reel on a line parallel to one side of a mounting frame. A pair of vacuum chambers are positioned in side by side relationship below the reels and parallel to the line. The vacuum columns are oriented in a head and tail manner (i.e., the open end of each vacuum column is positioned adjacent to the closed end of the other). A more detailed discussion of this type of configuration is given in U.S. Pat. Nos. 3,863,863 and 3,952,968. Due to the orientation of the vacuum columns the effective length of tape which the capstan moves increases. As the effective length of tape increases, so does the mass. The net result is that the system throughput is degraded since the acceleration of the capstan is reduced due to the increased mass of tape.